A non-directional capacitive microphone has been employed for a microphone to be housed in a housing of such as cassette type magnetic tape recorder. In this case, it has been usual to encase the microphone in a box or casing of such as aluminum. In arranging the non-directional capacitive microphone encased by the aluminum box in the casing, it has been also usual to make the rear portion and the peripheral side portion of the casing air-tight to prevent the microphone from picking up noises such as mechanical sounds generated by motor and/or vibrations of mechanical components disposed behind the microphone.
However, when the casing of the microphone is air-tightly sealed, it becomes impossible to regulate air pressures in the both sides of a diaphragm in the casing, causing the frequency characteristics of the microphone to be degraded. In order to resolve this problem, it has been usual to provide at least one hole in the rear wall or the peripheral side wall of the microphone casing, leaving the noise pick-up problem as it is.